This invention in one aspect relates to tuneable bandpass filters and, more particularly, to filters for use in a variety of equipments including spectrum analyzers and sine wave oscillators, but principally flow rate signal processors. In other aspects, my invention relates to unique trigger circuits for use in such processors.
In order to measure the flow rate of fluid in a conduit, it is common to utilize a Rodely bluff body to generate vortex shedding as described in U.S. Pat. No. 3,572,117 issued on Mar. 23, 1971, and assigned to the assignee hereof. The vortex shedding, which is oscillatory in nature, is in turn detected by a suitable sensor such as a thermistor, a shuttle or a diaphragm. The detector converts a parameter (e.g., temperature, pressure) variation of the fluid into an electrical signal. Typically, this signal is a composite of a low amplitude, high frequency fluid flow signal superimposed on a higher amplitude, random noise signal of lower frequency. The noise signal is generally not of interest.
Various types of electronic equipment have been proposed by the prior art to process the composite electrical signal and thereby measure flow rate. In many applications the flowmeter has been highly constrained by the high cost of such equipment. One system employs a series combination of a pre-amplifier, a filter and a schmitt trigger. The pre-amplifier incorporates frequency compensation which modifies the effective gain at various frequencies. The gain contouring is specific and is experimentally derived to get the best accuracy from the trigger circuit. Likewise, the signal passes through a fixed bandpass filter, one which has to be specifically designed for each application (e.g., each type of fluid or range of possible flow rates). In this type of equipment the filter bandwidth (f.sub.max :f.sub.min) typically has to be less than 30:1 even for a bluff body-sensor arrangement (meter) which produces high signal quality; and, of course, has to be much narrower for poor signal quality meters. In addition, the equipment has to be adjusted, sometimes critically, to achieve adequate accuracy.
To avoid having to redesign the filter for each different frequency (flow rate) range, the prior art has suggested that a tracking filter be substituted for the fixed bandpass filter. These filters fall into two categories: electrically and mechanically tuneable. Electrically tuneable filters depend on electrically variable components such as light sensitive resistors, FETs (Field Effect Transistors) and voltage variable capacitors as tuning elements. All of these electrical components are notoriously nonlinear and temperature sensitive. In addition, a tuneable filter employing FETs, for example, has two other disadvantages: it is very slow and hence cannot be utilized in most applications which require response to dynamic flow rate changes; and it is very expensive due to the need for matched FETs. A mechanically tuneable filter, on the other hand, is cumbersome, complex and requires sophisticated electronics to interface with the mechanics.
Another development in fluid flow detection is generally referred to as a signal processor and typically has two models, one for measuring liquid flow rates and the other for gas flow. This inherently causes a problem in measuring the flow rate of high pressure gas which has a signal quality between liquid and gas. The basic processor solves the tuning range problem by switching whole sections of circuitry on or off, according to flowrate, but is a precision system, quite complex and difficult to repair.
It is therefore one object of my invention to provide flowmetering equipment which operates over a relatively wide range of signal quality, frequency, signal amplitude and noise conditions.
It is another object of my invention to provide such equipment utilizing a building block approach to circuitry which is simple to repair and modify, and which employs low-cost, non-precision components.
It is yet another object of my invention to provide a unique tuneable bandpass filter for use in such equipment, as well as in other equipments not necessarily related to flowmetering.
It is still another object of my invention to provide a unique trigger circuit for detecting the filter output, especially in flowmetering applications.